A spark plug used for ignition of an internal combustion engine such as a gasoline engine generally includes a center electrode, an insulator provided externally of the center electrode, a metallic shell provided externally of the insulator, and a ground electrode attached to the metallic shell with a gap (discharge gap) formed between the ground electrode and the center electrode to allow spark discharge to occur therebetween (the ground electrode is also referred to as an “outer electrode”). In the following description, the side toward the gap is referred to as the “front side” of the center electrode or the ground electrode, and the side opposite to the “front side” is referred to as the “rear side.”
In known spark plugs, at least one of the center electrode and the ground electrode (hereinafter may be collectively referred to simply as “electrodes”) includes a cover portion formed of a prescribed material (e.g., nickel or a nickel alloy) and a core portion formed of a material (e.g., copper) having a thermal expansion coefficient different from that of the cover portion and covered with the cover portion (see, for example, Patent Documents 1 and 2). In such a spark plug, when a material having high thermal conductivity is selected as the material of the core portion, the heat transfer performance of the electrode can be improved.